1. Field of the Invention
The present invention relates to a method for synchronizing the coding and decoding during the transmission of digital, coded data from a transmitter to a receiver, a sequence of synchronizing signals being generated at a first clock rate at the transmitter and periodically being inserted into a data stream which is transmitted at a second clock rate, plus a system for carrying out this method.
2. Description of the Prior Art
Facilities for coding are preferably used if information which is transmitted along easily accessible transmission paths should be intelligible only for a certain receiver or for a specified circle of receivers. Known examples are radio links which can be monitored with technical means which are accessible to anybody.
The signal to be coded is coded in the transmitter in a code modulator which is controlled by a code generator. The output signal of the code modulator, which is now coded, is transmitted to the receiver and the transmission path can be, for example, a telephone line or a radio link. In the receiver, the received signal is fed to a code demodulator which is also controlled by a code generator. The output signal from the code demodulator is exactly identical to the original signal when the same code has been set at the two code generators and the code computers are running in synchronism.
In a known design of a coding facility for speech signals, the speech signal, which is present at the transmitter in an analog form, is first converted in an analog/digital converter into a binary data stream which is coded by the modulo 2 addition of a binary pseudo random sequence generated by the code generator. At the receiver, decoding is done by further modulo 2 addition of the same pseudo random sequence. The resulting sum signal is used to recover the analog speech signal by means of digital/analog conversion.
In a known design of a code computer, the secret pseudo random sequence is generated from a pseudo random sequence (source signal), which is not secret in itself, in a sequential logic system the sequences of which are a function of the secret code. The source signal generator used can be a random generator containing a shift register and an EXCLUSIVE OR gate the two inputs of which are each connected to a storage location and the output of which is or are connected to the input of the shift register. If the same initial signal sequence is read into the shift registers of two random generators of this type, which are constructed in the same way, and if the generators are operated synchronously, the same signals will appear at each of the successive operating clock pulses at the outputs of the EXCLUSIVE OR gates.
For synchronous operation of the source signal generators in the transmitter and receiver unit, each unit contains a crystal-controlled oscillator which functions as the clock generator. The clock generator in the receiver unit is associated with a phase control loop which secures frequency and phase tracking. Different methods are customarily used for setting the same signal sequences of the source signal generators.
During start synchronization, at the beginning of a transmission, a coherent sequence of synchronizing signals is transmitted, during continuous synchronizing synchronizing signals are periodically inserted into the stream of coded signals transmitted, which synchronizing signals sequence or periodic synchronizing signals, respectively, are read into the shift register of the source signal generator in the receiver unit. The start synchronization makes it possible rapidly to adjust the tracking of the source signal generator in the receiver unit but has the disadvantage that in the case where a receiver unit has been switched in only after the synchronizing signal sequence has been sent out, the tracking of the source signal generator cannot be set or can only be set when a further synchronizing signal sequence is sent out, that the adjustment of tracking and the tracking of the source signal generator itself can be disturbed by third parties and that after a disturbance and independently of its cause, the tracking of the source signal generator and thus the correct decoding of received coded information remains interrupted until a further synchronizing signal sequence is sent out. In contrast, continuous synchronization makes it possible to switch in one or several receiver units even after a transmission has begun, and the source signal generator in the receiver unit can recover its tracking after a disturbance. These advantages of continuous synchronization must be compared with the disadvantage that the adjusting of the tracking of the source signal generator in the receiver unit takes much longer than in the case of start synchronization. If synchronization is lost during transmission, for example because of fading or phase shifts in the signal sequence, resynchronizing takes a relatively long time.